1. Field of the Invention
The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with an apparatus and method for measuring the spring force imposed on a fuel rod when disposed through a cell in the support grid of the fuel assembly.
2. Description of the Prior Art
In most nuclear reactors, the reactor core is comprised of a large number of elongated fuel assemblies. Conventional designs of these fuel assemblies include a plurality of fuel rods held in an organized array by a plurality of grids spaced axially along the fuel assembly length and attached to a plurality of elongated control rod guide thimbles of the fuel assembly. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the guide thimbles which extend slightly above the below the ends of the fuel rods.
The grids as well known in the art are used to precisely maintain the spacing between the fuel rods in the reactor core, prevent rod vibration, provide lateral support for the fuel rods, and, to some extent, frictionally retain the rods against longitudinal movement. Conventional designs of grids include a multipicity of interleaved straps having an egg-crate configuration designed to form cells which individually accept the fuel rods and control rod guide thimbles. The cells of each grid which accept and support the fuel rods at a given axial location therealong typically use relatively resilient springs and relatively rigid protrusions (called dimples) formed into the metal of the interleaved straps. The springs and dimples of each grid cell frictionally engage or contact the respective fuel rod extending through the cell. Additionally, outer straps are attached together and peripherally enclose the grid straps to impart strength and rigidity to the grid.
In view that the opeation of the reactor core involves irradiation of the fuel rods, it is desirable that the supporting forces imposed by the grid on the fuel rods fall within such a range that the grid adequately supports the fuel rods but does not promote lengthwise distortion thereof over time. Also, in view that the grids are irradiated along with the fuel rods they support, the grids inherently degrade somewhat over time as a result. In particular, notwithstanding improvements in grid construction, such as illustrated and described in U.S. Pat. No. 4,474,730 issued Oct. 2, 1984 and assigned to the assignee of the present invention, over the life of the fuel assembly the metal of the interleaved straps forming the grid, and thereby the springs and dimples formed therein, are subject to stress relaxation due to irradiation.
In light of the above conditions, it is necessary to initially carry out grid spring force measurements during the manufacture of the grid. Then, although the strap material does not ordinarily suffer a significant loss of resiliency when exposed to intense and prolonged irradiation, it is also desirable to carry out periodic checks of the forces imposed by the springs on the fuel rods during the life of the grid in order to monitor and detect the degree of degradation of the grid in terms of its ability to continue to provide adequate fuel rod support.
One present technique for initially measuring grid forces during manufacture is to select a typical spring taken from a strip of formed strap material and characterize it by force/deflection curves. Other techniques include the use of a solid bar acting on a grid spring in a grid assembly to provide similar force/deflection curves which take into account some of the spring/cell boundary conditions. From the above techniques, grid spring forces for a given size fuel rod are interpolated from a set of curves based on statistically averaged values. However, with these techniques it is not possible to evaluate by direct means the interaction of the dimples and strap joints opposing the forces imposed by the grid spring, nor the flexibility of the grid strap as an individual cell wall of a total grid assembly containing numerous cells.
Another techniques for measuring grid spring forces is illustrated and described in U.S. Pat. No. 4,246,783 to Steven et al. The Steven et al measuring device uses a flexible beam on the force measuring plug having a diameter equal to the diameter of the fuel rod to be inserted into the spacer gird. A strain gage is disposed on the flexible beam and electrically picks up bending of the flexible beam as the device is inserted into the grid cell. However, it is necessary to have available different plugs with the proper dimensions for measuring different sizes of fuel rod diameters and different spacer geometries.
Consequently, a need exists for a technique to measure grid spring forces in a manner which more nearly replicates real-life conditions, that is to say, the actual forces experienced by fuel rods as they are supported within the cells of the grid by the springs and dimples and to be able to carry out such measurements with relative ease, speed and accuracy,